3.17 GCC Developer Options

This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; that print statistics such as memory use and execution time; and that print information about GCC's configuration, such as where it searches for libraries. You should rarely need to use any of these options for ordinary compilation and linking tasks.

-dletters

-fdump-rtl-pass

-fdump-rtl-pass=filename

Says to make debugging dumps during compilation at times specified by letters
. This is used for debugging the RTL-based passes of the compiler. The file names for most of the dumps are made by appending a pass number and a word to the dumpname
, and the files are created in the directory of the output file. In case of =filename
option, the dump is output on the given file instead of the pass numbered dump files. Note that the pass number is assigned as passes are registered into the pass manager. Most passes are registered in the order that they will execute and for these passes the number corresponds to the pass execution order. However, passes registered by plugins, passes specific to compilation targets, or passes that are otherwise registered after all the other
passes are numbered higher than a pass named "final", even if they are executed earlier. dumpname
is generated from the name of the output file if explicitly specified and not an executable, otherwise it is the basename of the source file.

Some -dletters
switches have different meaning when -E
is used for preprocessing. See Preprocessor Options
, for information about preprocessor-specific dump options.

Debug dumps can be enabled with a -fdump-rtl
switch or some -d
option letters
. Here are the possible letters for use in pass
and letters
, and their meanings:

-fdump-rtl-alignments

Dump after branch alignments have been computed.

-fdump-rtl-asmcons

Dump after fixing rtl statements that have unsatisfied in/out constraints.

-fdump-rtl-auto_inc_dec

Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.

-fdump-rtl-barriers

Dump after cleaning up the barrier instructions.

-fdump-rtl-bbpart

Dump after partitioning hot and cold basic blocks.

-fdump-rtl-bbro

Dump after block reordering.

-fdump-rtl-btl1

-fdump-rtl-btl2

-fdump-rtl-btl1
and -fdump-rtl-btl2
enable dumping after the two branch target load optimization passes.

-fdump-rtl-bypass

Dump after jump bypassing and control flow optimizations.

-fdump-rtl-combine

Dump after the RTL instruction combination pass.

-fdump-rtl-compgotos

Dump after duplicating the computed gotos.

-fdump-rtl-ce1

-fdump-rtl-ce2

-fdump-rtl-ce3

-fdump-rtl-ce1
, -fdump-rtl-ce2
, and -fdump-rtl-ce3
enable dumping after the three if conversion passes.

-fdump-rtl-cprop_hardreg

Dump after hard register copy propagation.

-fdump-rtl-csa

Dump after combining stack adjustments.

-fdump-rtl-cse1

-fdump-rtl-cse2

-fdump-rtl-cse1
and -fdump-rtl-cse2
enable dumping after the two common subexpression elimination passes.

-fdump-rtl-dce

Dump after the standalone dead code elimination passes.

-fdump-rtl-dbr

Dump after delayed branch scheduling.

-fdump-rtl-dce1

-fdump-rtl-dce2

-fdump-rtl-dce1
and -fdump-rtl-dce2
enable dumping after the two dead store elimination passes.

-fdump-rtl-eh

Dump after finalization of EH handling code.

-fdump-rtl-eh_ranges

Dump after conversion of EH handling range regions.

-fdump-rtl-expand

Dump after RTL generation.

-fdump-rtl-fwprop1

-fdump-rtl-fwprop2

-fdump-rtl-fwprop1
and -fdump-rtl-fwprop2
enable dumping after the two forward propagation passes.

-fdump-rtl-gcse1

-fdump-rtl-gcse2

-fdump-rtl-gcse1
and -fdump-rtl-gcse2
enable dumping after global common subexpression elimination.

Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.

-fdump-unnumbered

When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use diff on debugging dumps for compiler invocations with different options, in particular with and without -g
.

-fdump-unnumbered-links

When doing debugging dumps (see -d
option above), suppress instruction numbers for the links to the previous and next instructions in a sequence.

-fdump-translation-unit(C++ only)

-fdump-translation-unit-options(C++ only)

Dump a representation of the tree structure for the entire translation unit to a file. The file name is made by appending .tu
to the source file name, and the file is created in the same directory as the output file. If the ‘-options
’ form is used, options
controls the details of the dump as described for the -fdump-tree
options.

-fdump-class-hierarchy(C++ only)

-fdump-class-hierarchy-options(C++ only)

Dump a representation of each class's hierarchy and virtual function table layout to a file. The file name is made by appending .class
to the source file name, and the file is created in the same directory as the output file. If the ‘-options
’ form is used, options
controls the details of the dump as described for the -fdump-tree
options.

-fdump-ipa-switch

Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. The following dumps are possible:

Print on stderr
the list of optimization passes that are turned on and off by the current command-line options.

-fdump-statistics-option

Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a suffix ending in ‘.statistics
’ to the source file name, and the file is created in the same directory as the output file. If the ‘-option
’ form is used, ‘-stats
’ causes counters to be summed over the whole compilation unit while ‘-details
’ dumps every event as the passes generate them. The default with no option is to sum counters for each function compiled.

-fdump-tree-all

-fdump-tree-switch

-fdump-tree-switch-options

-fdump-tree-switch-options=filename

Control the dumping at various stages of processing the intermediate language tree to a file. The file name is generated by appending a switch-specific suffix to the source file name, and the file is created in the same directory as the output file. In case of =filename
option, the dump is output on the given file instead of the auto named dump files. If the ‘-options
’ form is used, options
is a list of ‘-
’ separated options which control the details of the dump. Not all options are applicable to all dumps; those that are not meaningful are ignored. The following options are available

‘ address
’

Print the address of each node. Usually this is not meaningful as it changes according to the environment and source file. Its primary use is for tying up a dump file with a debug environment.

‘ asmname
’

If DECL_ASSEMBLER_NAME
has been set for a given decl, use that in the dump instead of DECL_NAME
. Its primary use is ease of use working backward from mangled names in the assembly file.

‘ slim
’

When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. Only dump such items when they are directly reachable by some other path.

When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.

‘ raw
’

Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.

‘ details
’

Enable more detailed dumps (not honored by every dump option). Also include information from the optimization passes.

‘ stats
’

Enable dumping various statistics about the pass (not honored by every dump option).

‘ blocks
’

Enable showing basic block boundaries (disabled in raw dumps).

‘ graph
’

For each of the other indicated dump files (-fdump-rtl-pass
), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot
. Each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

This option currently only works for RTL dumps, and the RTL is always dumped in slim form.

To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.

Invoke GCC with -fdump-passes
and in the stderr
output look for a code that corresponds to the pass you are interested in. For example, the codes tree-evrp
, tree-vrp1
, and tree-vrp2
correspond to the three Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.

To enable the creation of the dump file, append the pass code to the -fdump-
option prefix and invoke GCC with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp
option. Optionally, you may specify the name of the dump file. If you don't specify one, GCC creates as described below.

Find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter ‘t
’ for tree passes (and the letter ‘r
’ for RTL passes), and finally the pass code. For example, the Early VRP pass dump might be in a file named myfile.c.038t.evrp
in the current working directory. Note that the numeric codes are not stable and may change from one version of GCC to another.

-fopt-info

-fopt-info-options

-fopt-info-options=filename

Controls optimization dumps from various optimization passes. If the ‘-options
’ form is used, options
is a list of ‘-
’ separated option keywords to select the dump details and optimizations.

The options
can be divided into two groups: options describing the verbosity of the dump, and options describing which optimizations should be included. The options from both the groups can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line.

The following options control the dump verbosity:

‘ optimized
’

Print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.

‘ missed
’

Print information about missed optimizations. Individual passes control which information to include in the output.

‘ note
’

Print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.

One or more of the following option keywords can be used to describe a group of optimizations:

‘ ipa
’

Enable dumps from all interprocedural optimizations.

‘ loop
’

Enable dumps from all loop optimizations.

‘ inline
’

Enable dumps from all inlining optimizations.

‘ omp
’

Enable dumps from all OMP (Offloading and Multi Processing) optimizations.

‘ vec
’

Enable dumps from all vectorization optimizations.

‘ optall
’

Enable dumps from all optimizations. This is a superset of the optimization groups listed above.

If options
is omitted, it defaults to ‘optimized-optall
’, which means to dump all info about successful optimizations from all the passes.

If the filename
is provided, then the dumps from all the applicable optimizations are concatenated into the filename
. Otherwise the dump is output onto stderr
. Though multiple -fopt-info
options are accepted, only one of them can include a filename
. If other filenames are provided then all but the first such option are ignored.

Note that the output filename
is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr
should be used instead.

In the following example, the optimization info is output to stderr
:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs missed optimization report from all the passes into missed.all
, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr
. Note that -fopt-info-vec-missed
is equivalent to -fopt-info-missed-vec
.

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt
.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss
and loop.opt
are in conflict since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss
is produced which contains dumps from the vectorizer about missed opportunities.

-fsched-verbose=n

On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.

For n
greater than zero, -fsched-verbose
outputs the same information as -fdump-rtl-sched1
and -fdump-rtl-sched2
. For n
greater than one, it also output basic block probabilities, detailed ready list information and unit/insn info. For n
greater than two, it includes RTL at abort point, control-flow and regions info. And for n
over four, -fsched-verbose
also includes dependence info.

-fenable-kind-pass

-fdisable-kind-pass=range-list

This is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead.

-fdisable-ipa-pass

Disable IPA pass pass
. pass
is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

-fdisable-rtl-pass

-fdisable-rtl-pass=range-list

Disable RTL pass pass
. pass
is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list
is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid
falls within one of the specified ranges, the pass
is disabled for that function. The uid
is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes
.

-fdisable-tree-pass

-fdisable-tree-pass=range-list

Disable tree pass pass
. See -fdisable-rtl
for the description of option arguments.

-fenable-ipa-pass

Enable IPA pass pass
. pass
is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.

This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed
option to produce reproducibly identical object files.

The string
can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32).

The string
should be different for every file you compile.

-save-temps

-save-temps=cwd

Store the usual “temporary” intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c
with -c -save-temps
produces files foo.i
and foo.s
, as well as foo.o
. This creates a preprocessed foo.i
output file even though the compiler now normally uses an integrated preprocessor.

When used in combination with the -x
command-line option, -save-temps
is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps
.

If you invoke GCC in parallel, compiling several different source files that share a common base name in different subdirectories or the same source file compiled for multiple output destinations, it is likely that the different parallel compilers will interfere with each other, and overwrite the temporary files. For instance:

may result in foo.i
and foo.o
being written to simultaneously by both compilers.

-save-temps=obj

Store the usual “temporary” intermediate files permanently. If the -o
option is used, the temporary files are based on the object file. If the -o
option is not used, the -save-temps=obj
switch behaves like -save-temps
.

Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).

Without the specification of an output file, the output looks like this:

# cc1 0.12 0.01
# as 0.00 0.01

The first number on each line is the “user time”, that is time spent executing the program itself. The second number is “system time”, time spent executing operating system routines on behalf of the program. Both numbers are in seconds.

With the specification of an output file, the output is appended to the named file, and it looks like this:

0.12 0.01 cc1 options
0.00 0.01 as options

The “user time” and the “system time” are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options.

-fdump-final-insns[=file]

Dump the final internal representation (RTL) to file
. If the optional argument is omitted (or if file
is .
), the name of the dump file is determined by appending .gkd
to the compilation output file name.

-fcompare-debug[=opts]

If no error occurs during compilation, run the compiler a second time, adding opts
and -fcompare-debug-second
to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.

If the equal sign is omitted, the default -gtoggle
is used.

The environment variable GCC_COMPARE_DEBUG
, if defined, non-empty and nonzero, implicitly enables -fcompare-debug
. If GCC_COMPARE_DEBUG
is defined to a string starting with a dash, then it is used for opts
, otherwise the default -gtoggle
is used.

-fcompare-debug=
, with the equal sign but without opts
, is equivalent to -fno-compare-debug
, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG
from taking effect.

To verify full coverage during -fcompare-debug
testing, set GCC_COMPARE_DEBUG
to say -fcompare-debug-not-overridden
, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG
to ‘-w%n-fcompare-debug not overridden
’ will do.

-fcompare-debug-second

This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug
, along with options to silence warnings, and omitting other options that would cause side-effect compiler outputs to files or to the standard output. Dump files and preserved temporary files are renamed so as to contain the .gk
additional extension during the second compilation, to avoid overwriting those generated by the first.

When this option is passed to the compiler driver, it causes the first
compilation to be skipped, which makes it useful for little other than debugging the compiler proper.

-gtoggle

Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug
.

-fvar-tracking-assignments-toggle

Toggle -fvar-tracking-assignments
, in the same way that -gtoggle
toggles -g
.

-Q

Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.

-ftime-report

Makes the compiler print some statistics about the time consumed by each pass when it finishes.

-ftime-report-details

Record the time consumed by infrastructure parts separately for each pass.

-fira-verbose=n

Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n
is greater or equal to 10, the dump output is sent to stderr using the same format as n
minus 10.

-flto-report

Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto
).

Disabled by default.

-flto-report-wpa

Like -flto-report
, but only print for the WPA phase of Link Time Optimization.

-fmem-report

Makes the compiler print some statistics about permanent memory allocation when it finishes.

-fmem-report-wpa

Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.

-fpre-ipa-mem-report

-fpost-ipa-mem-report

Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.

-fprofile-report

Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.

-fstack-usage

Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su
to the auxname
. auxname
is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:

The name of the function.

A number of bytes.

One or more qualifiers: static
, dynamic
, bounded
.

The qualifier static
means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.

The qualifier dynamic
means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded
is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.

-fstats

Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team.

-fdbg-cnt-list

Print the name and the counter upper bound for all debug counters.

-fdbg-cnt=counter-value-list

Set the internal debug counter upper bound. counter-value-list
is a comma-separated list of name
:value
pairs which sets the upper bound of each debug counter name
to value
. All debug counters have the initial upper bound of UINT_MAX
; thus dbg_cnt
returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:10,tail_call:0
, dbg_cnt(dce)
returns true only for first 10 invocations.

-print-file-name=library

Print the full absolute name of the library file library
that would be used when linking—and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.

-print-multi-directory

Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX
.

-print-multi-lib

Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ‘;
’, and each switch starts with an ‘@
’ instead of the ‘-
’, without spaces between multiple switches. This is supposed to ease shell processing.

-print-multi-os-directory

Print the path to OS libraries for the selected multilib, relative to some lib
subdirectory. If OS libraries are present in the lib
subdirectory and no multilibs are used, this is usually just .
, if OS libraries are present in libsuffix
sibling directories this prints e.g. ../lib64
, ../lib
or ../lib32
, or if OS libraries are present in lib/subdir
subdirectories it prints e.g. amd64
, sparcv9
or ev6
.

-print-multiarch

Print the path to OS libraries for the selected multiarch, relative to some lib
subdirectory.

-print-prog-name=program

Like -print-file-name
, but searches for a program such as cpp
.

-print-libgcc-file-name

Same as -print-file-name=libgcc.a
.

This is useful when you use -nostdlib
or -nodefaultlibs
but you do want to link with libgcc.a
. You can do:

gcc -nostdlib files
... `gcc -print-libgcc-file-name`

-print-search-dirs

Print the name of the configured installation directory and a list of program and library directories gcc
searches—and don't do anything else.

This is useful when gcc
prints the error message ‘installation problem, cannot exec cpp0: No such file or directory
’. To resolve this you either need to put cpp0
and the other compiler components where gcc
expects to find them, or you can set the environment variable GCC_EXEC_PREFIX
to the directory where you installed them. Don't forget the trailing ‘/
’. See Environment Variables
.

-print-sysroot

Print the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the --sysroot
option, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing.

-print-sysroot-headers-suffix

Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not configured with such a suffix—and don't do anything else.

Print the compiler version (for example, 3.0
, 6.3.0
or 7
)—and don't do anything else. This is the compiler version used in filesystem paths, specs, can be depending on how the compiler has been configured just a single number (major version), two numbers separated by dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version).